1. Field of the Invention
The present invention relates to a cross dichroic prism used for decomposing source light into three primary colors and combining modulated individual color light components which are outputted from light valves in a full-color projector, a method of making the same, and a full-color projector using the same.
2. Related Background Art
Widely used in color-combining and decomposing systems in so-called three-sheet type full-color projectors is a quadrangular dichroic prism in which four isosceles right triangular prisms are assembled together at their rectangular portions, while a dichroic film is formed on thus pasted faces so as to yield an X-shaped cross section.
When this dichroic prism is used in a color-combining system, monochromatic light images of red (R), blue (B), and green (G) are respectively made incident on three faces among four side faces of the quadrangular prism and reflected by or transmitted through the dichroic film disposed at a center thereof, so as to be emitted out of the remaining one side face, thereby forming a combined light image. Here, when adjacent triangular prisms among the four triangular prisms constituting the dichroic prism have refractive indexes totally different from each other, refraction may occur between the adjacent prisms. As a result, emitted light images may not align with each other, thereby generating color shift.
For preventing the color shift, it is necessary to reduce the refractive index difference among the triangular prisms. One of such techniques is disclosed in Japanese Patent Application Laid-Open No. 6-331807.
In this technique, the refractive index difference between at least two sets of triangular prisms adjacent to each other is suppressed to 0.0005 or less. It discloses that, for this purpose, using prisms made in the same lot is important. It also discloses that, as a result, the optical axis deviation among the individual colors, i.e., image deviation, in a quadrangular dichroic prism whose sides each have a length of 40 mm can be suppressed to 13 xcexcm, which is about ⅓ of the pixel size of normally employed light valves, i.e., 40 xcexcm.
As projected pictures have become finer, light valves employed in projectors have been reducing their individual pixel sizes. On the other hand, in order to respond to wider pictures, dichroic prisms themselves have been increasing their sizes. As a result, the refractive index difference that has been permissible in the dichroic prisms used for projectors for projecting conventional TV images may yield a critical optical axis deviation in such dichroic prisms that are used for projectors for projecting highly fine wide pictures.
For further reducing the optical axis deviation, it is necessary to further suppress the refractive index difference between the adjacent triangular prisms. The inventors have found, however, it difficult to suppress the refractive index difference of prisms to a level much smaller than 0.0005 even when they are manufactured in the same lot under the same processing condition from the same glass material. Even if there exists a combination of prisms in the same lot whose refractive index difference is small enough to satisfy the condition where the optical axis deviation is minimized, it takes time and labor to find out such a combination, which is unpractical.
The inventors, however, have found it possible to homogenize the refractive index distribution with a very high accuracy within the same member by controlling the conditions for making prism members, annealing, and the like. The present invention utilizes this finding; and relates to a dichroic prism which prevents image deviation and blurring from occurring due to the optical axis deviation, a method of making the same, and a projector utilizing the same.
Namely, the cross dichroic prism in accordance with the present invention comprises four triangular prisms having isosceles right triangular cross sections identical in shape, opposing side faces thereof being bonded and secured together with rectangular portions thereof facing each other; and a dichroic film having an X-shaped cross section formed between the bonding faces of the triangular prisms; wherein, of the four triangular prisms, a given prism and at least one of the prisms adjacent thereto have a refractive index difference therebetween of 0.00005 or less, and the remaining two prisms opposed thereto have a refractive index difference therebetween of 0.00005 or less.
Among the four triangular prisms, for example, a given prism and at least one of the prisms adjacent thereto are formed by processing a single prism member, whereas the remaining two triangular prisms opposed thereto are formed by processing the same or another single prism member.
It is sufficiently possible to keep a refractive index difference of 0.00005 or less within a single prism member, though its manufacturing condition has to be controlled strictly. The dichroic prism thus obtained by assembling two sets of triangular prisms each comprising two triangular prisms processed from the same prism member is suitable for a projector for projecting highly fine wide pictures.
On the other hand, the method of making a cross dichroic prism in accordance with the present invention comprises a first step of preparing two prism members each having an isosceles right triangular cross section with a cross-sectional area twice as much as that of each of four triangular prisms constituting the cross dichroic prism; a second step of forming a first dichroic film on the hypotenuse face of one of the prism members; a third step of forming a cemented member shaped like a regular quadrangular prism by bonding and securing the hypotenuse faces of the prism members together with an adhesive; a fourth step of cutting the cemented member at a plane which is orthogonal to the dichroic film and connects the opposing right angles of the two prism members, thereby forming two cemented members identically shaped like an isosceles right triangular prism; a fifth step of forming a second dichroic film on the hypotenuse face of one of the triangular-prism-shaped cemented members; and a sixth step of bonding and securing the hypotenuse faces of the triangular-prism-shaped cemented members together with an adhesive so as to form a prism having a cross dichroic film with an X-shaped cross section at a center thereof.
As a consequence, a dichroic prism in which the above-mentioned adjacent triangular prisms have a small refractive index difference therebetween can be obtained securely and easily.
The full-color projector in accordance with the present invention comprises the above-mentioned cross dichroic prism in accordance with the present invention. As a consequence, the optical axes of the respective light images of three primary colors can be aligned with each other with a high accuracy, whereby the projector is suitable for projecting highly fine wide pictures.
The present invention will be more fully understood from the detailed description given hereinbelow and the accompanying drawings, which are given by way of illustration only and are not to be considered as limiting the present invention.
Further scope of applicability of the present invention will become apparent from the detailed description given hereinafter. However, it should be understood that the detailed description and specific examples, while indicating preferred embodiments of the invention, are given by way of illustration only, since various changes and modifications within the spirit and scope of the invention will be apparent to those skilled in the art from this detailed description.